Photonic crystals (PCs) are attractive optical materials for controlling and manipulating the flow of light. Specifically, PCs are composed of periodic dielectric or metallic-dielectric nanostructures and affect the propagation of electromagnetic waves by defining allowed and forbidden photonic bands. When a structural defect is introduced in the PCs, a photon-localized state can be created in the photonic band gap and the electrical field around the defect can be confined and enhanced. Control of defect modes by PC structures is becoming a key technology for many new photonic devices, like photonic crystal fibers, photonic chips, low-threshold lasers, and optical biosensors.
Also, surface plasmons (SPs) have attracted tremendous interest over the last two decades, both from a fundamental-physics perspective and as highly sensitive devices for optical detection of small biological or chemical entities. SPs are waves that propagate along the surface of a conductor, usually a metal film, and they are essentially p polarized light waves that are trapped on the surface because of their interaction with the free electrons of the metal. This part of light is further transferred into heat within the metal film and gets lost. This property is the basic mechanism of surface plasmon resonance (SPR) based biosensor. The surface plasmon modes can be excited in the configuration by phase matching. The field intensity is enhanced in the metal, but decays both in the dielectric layer (substrate) and the surrounding medium. The excited surface plasmon mode can be characterized by a (very broad) resonance dip in the reflectance spectrum since this part of light is resonant and absorbed in the very lossy metal film.
Besides bulk prism configuration in SPR sensors, optical waveguides, fibers, and gratings have also been developed to excite surface plasmon modes, and they offer advantages of miniaturization, a high degree of integration and remote sensing applications. However, phase matching in these structures can be difficult.